1. Field of the Invention
The present invention relates to processes for producing low nitrogen, essentially nitride-free chromium and chromium plus niobium-containing nickel-based alloys and the resulting chromium and nickel-based alloys.
2. Description of Related Art
The lifespan of rotating metal parts in aircraft engines is typically determined by fatigue cracking. In this process, cracks are initiated at certain nucleation sites within the metal and propagate at a rate related to the material characteristics and the stress to which the component is subjected. That, in turn, limits the number of cycles the part will withstand during its service life.
Clean melting production techniques developed for superalloys have given rise to the substantial elimination of oxide inclusions in such alloys to the extent that nowadays, fatigue cracks are mainly originated on structural features, for example, on grain boundaries or clusters of primary precipitates such as carbides and nitrides.
It has been found that the primary nitride particles formed during the solidification of alloy 718 (see, alloy 718 specifications (AMS 5662 and API 6A 718) which are incorporated herein by reference)—which is one of the main alloys utilized in the production of aircraft engine rotating parts and for oil and gas drilling and production equipment—are pure TiN (titanium nitride) and that the precipitation of primary Nb—TiC (niobium-titanium carbide) occurs by heterogeneous nucleation over the surface of the TiN particles, thereby increasing the precipitate particle size. The particle size can be decreased by two means: either by lowering the carbon content as much as possible, or by lowering the nitrogen content.
Many commercial specifications for superalloys, stainless steel, and other specialty steels, establish minimum carbon content, usually in order to prevent grain boundary slipping at the service temperature. As a consequence, the only practical means to decrease particle size compositionally is to reduce the nitrogen content in the material as extensively as possible. In that way, in as much as the nitrides precipitate first, removing nitrogen supersedes the importance of removing carbon.
However, once the nitride precipitation is suppressed, the carbon content of the bulk liquid can also be decreased, due to the fact that no carbon will be consumed by precipitation around the nitride particle. This will lead to an improvement in eventual differences of densities between the interdendritic liquid at the solidification front and the bulk liquid. As a consequence, a lesser degree of segregation can be obtained which facilitates the production of ingots larger than the current standards in the industry, while still meeting all the properties and expected performance criteria in use.
Moreover, the development of this type of material provides substantial advantages in the production of single-crystal nickel-based superalloys. One of the main problems with that technology is to avoid the deleterious effect of the precipitation of titanium nitrides, since those particles become heterogeneous nuclei for dendrites that act as additional solidification fronts. That would create boundaries thereby preventing the casting from having a homogeneous structure. In Solidification and Precipitation in IN718, A. Mitchell and T. Wang, Superalloys 718. 625.706 and Various Derivations, Edited by E. A. Loria, TMS (The Minerals, Metals and Materials Society), 2001, it is reported that if nitride-free feedstock could be obtained, it would enable doubling the solidification rate vis-à-vis the solidification rate used to produce the same part with conventional material.